A coherent mechanistic rationalization of the role of water in aldol reactions employing aromatic aldehydes shows that the intrinsic kinetic effect of water within the catalytic cycle is a suppression of reaction rate. The presence of water suppresses formation of key intermediates within the cycle as well as reversibly and irreversibly formed spectator species such as oxazolidinones and oxapyrrolizidines, respectively. The net effect on the observed productivity of the reaction will depend on the balance between these two effects. This work highlights the complex role that water plays both on and off the catalytic cycle and the need to separate these effects to achieve mechanistic understanding.

Several zwitterionic iridium complexes based on chiral P,N-ligands with imidazoline or oxazoline donors and anionic tetraarylborate or aryltrifluoroborate substituents have been synthesized. The corresponding cationic analogues have also been prepared, to evaluate the effect of the covalent linkage between the anion and the cationic metal complex in catalytic reactions. The respective pairs of structurally analogous precatalysts have been compared for their efficacies in the asymmetric hydrogenation of unfunctionalized olefins. In most cases, the anionic derivatization has virtually no influence on the asymmetric induction of the iridium complex. This is in accordance with X-ray structural studies, which have shown that the chiral environment of the cationic metal center is not affected by the anionic substituent. Depending on the nature of the counterion employed, the zwitterionic catalysts proved to be significantly more reactive than their cationic counterparts in nonpolar solvents.

A series of monodentate neutral and anionic phosphorus ligands was synthesized and evaluated in the asymmetric rhodium-catalyzed hydrogenation of functionalized olefins by using either catalysts containing identical ligands or catalysts generated from mixtures of two different ligands. We expected that the combination of an anionic ligand with a neutral ligand would favor the formation of hetero over homo bis-ligand complexes due to charge repulsion. NMR spectroscopic studies confirmed that charge effects can indeed shift the equilibrium toward the hetero bis-ligand complexes. In several cases, the combination of a neutral phosphane with an anionic phosphane, one chiral and the other achiral, furnished significantly higher enantioselectivities than analogous mixtures of two neutral ligands. The best results were obtained with a mixture of an anionic phosphoramidite and a neutral phosphoric acid diester. It is supposed that in this case a hydrogen bond between the two ligands additionally stabilizes the hetero ligand combination.

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Zwitterionic Iridium Complexes with P,N-Ligands as Catalysts for the Asymmetric Hydrogenation of Alkenes. -A variety of imidazoline-, or oxazoline-based chiral iridium-P,N ligand complexes, e.g. IPOXA, IPOXB, is prepared and studied as catalyst in the hydrogenation of various olefins. Generally, better results are achieved with oxazoline complexes (concerning reactivity and enantioselectivity), irrespective of the ionic structure (betaine or salt). - (FRANZKE, A.; PFALTZ*, A.; Chem. Eur. J. 17 (2011) 15, 4131-4144, http://dx.doi.org/10.1002/chem.201003314 ; Dep. Chem., Univ. Basel, CH-4056 Basel, Switz.; Eng.) -Mischke 32-028

This is a study concerning ligand effects and catalyst formation in the asymmetric hydrogenation of olefins.

Mono-and bifunctional fluorinated borate building blocks were prepared in three to five steps with good to excellent overall yields. Compounds with both nucleophilic and electrophilic functionalities are presented, which can be used for the anionic derivatization of neutral molecules.Fluorinated tetraarylborates constitute an easily accessible class of weakly coordinating anions (WCA), 2 which have found widespread use in the synthesis of highly efficient single-site olefin polymerization catalysts. 3 In addition, they were successfully applied in catalytic asymmetric transformations such as the iridium-catalyzed hydrogenation of unfunctionalized alkenes 4 and DielsAlder reactions. 5 In most cases, the well-established representatives tetrakis(pentafluorophenyl)borate 6 or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAr F or TFPB) 7 were employed. Furthermore, related anions with four identical aryl moieties at the boron center have recently been developed. 8 However, there are only a few examples of borates having the composition [B(C 6 F 5 ) 3 (R)] -with different boron substituents, and only simple alkyl derivatives [R = Me, Et, i-Pr, n-Bu, CH 2 C(CH 3 ) 3 ] 9 and the phenyl analogue (R = Ph) 10 have been described in detail.In principle, the synthesis of [B(Ar) 3 (R)] -species can be accomplished via two synthetic pathways (Scheme 1), differing in the order of steps through which the two substituents at the boron center are introduced. 9,10 Scheme 1 Two pathways for the preparation of mixed tetraarylborates If the borane B(Ar) 3 is easily accessible, the desired borate can be obtained according to pathway A, in one step, by addition of the appropriate metal organyl compound. Alternatively, a neutral (RBX 2 ) or anionic intermediate (RBX 3 -) can be generated first and subsequently transformed into the borate by reaction with the corresponding aryl reagent according to pathway B.In the course of our studies concerning the effect of the counter-ion in iridium-catalyzed asymmetric hydrogenation reactions, 4,11 functionalized borate building blocks were required to prepare anionic derivatives of neutral ligands. In this report, we describe the synthesis of a series of functionalized borates bearing nucleophilic or electrophilic functional groups, which can be used for the anionic functionalization of neutral compounds. Figure 1 Functionalized borates with two different linkersInitial attempts to prepare compounds of type 1 with a benzyl linker derived from the commercially available tris(pentafluorophenyl)borane ( Figure 1) via pathway A, proved unsuccessful. The intermediate borates suffered from partial decomposition, presumably by proton-induced deborylation of the benzyl moiety, even under only slightly acidic conditions, for example during silica gel chromatography. Therefore, we turned our attention to the corresponding derivatives 2 with perfluorinated linkers. We assumed that the reduced p-basicity of these compounds would slow down the acid-driven cleavage of the carbon-boron bond and thus enable chr...